Digital printing machines can take on a variety of configurations. One common process is that of electrostatographic printing, which is carried out by exposing a light image of an original document to a uniformly charged photoreceptive member to discharge selected areas. A charged developing material is deposited to develop a visible image. The developing material is transferred to a medium sheet (paper) and heat fixed.
Another common process is that of direct to paper ink jet printing systems. In ink jet printing, tiny droplets of ink are sprayed onto the paper in a controlled manner to form the image. Other processes are well known to those skilled in the art.
The primary output product for a typical digital printing system is a printed copy substrate such as a sheet of paper bearing printed information in a specified format. Quite often, customer requirements necessitate that this output product be configured in various specialized arrangements ranging from stacks of collated loose printed sheets, to brief reports stapled together, to tabulated and bound booklets. The sheets of media, usually paper, are compiled, stapled, and ejected at the last stage of the job, in a region called a finisher.
Various external output devices have been designed for connection to a digital printing machine. The paper will exit the printing system and be passed to an external finishing device, wherein a critical parameter in such delivery is the capability to operate at process speed so as to not inhibit the function of the printing machine.
Finishing procedures, such as sorting, collating, stapling and ejecting, require the movement of mechanical components. In state of the art digital printing machines, it is common to have a quantity of sets in a job stream which require various sorts of finishing activities. In order to accommodate multiple sets, each set in the stream is typically held or delayed until the finishing activity of the preceding set has been completed. Moreover, it is often necessary to slow the output speed of the printing machine so as not to exceed the rate at which the external device, or finisher, can receive and process sets of output documents for producing the final output product. These finishing delay times detract from the overall productivity of the printing system.
Sheet registration must be carried out before stapling and ejecting sets are accomplished. Certain high speed production finishers utilize a scuffer mechanism during stacking to register the leading edge of the sheets by driving them into a vertical plate. In addition, the sheets are registered laterally by side tampers. The scuffing (process direction registration) and tamping (cross process registration) actions occur sequentially. The scuffer must lift prior to tamping to allow free lateral movement of the sheet. The scuffer then lowers to receive the next incoming sheet. An example of this registration system is found in Schwenk, U.S. Pat. No. 6,856,785, filed on Dec. 22, 2003. One problem with this method is that it slows productivity, because the in-line registration and the lateral registration are performed consecutively. Another problem with this method is that during the tamping process, the process direction registration may deteriorate since the sheets are no longer held by the scuffer in the process direction.
Mandel, U.S. Pat. No. 5,120,047, filed on Feb. 7, 1991, shows a scuffer wheel mechanism disposed at an angle to the process direction. The scuffer drives the paper against a first wall in the process direction, and against a second wall in the cross process direction. A problem with this type of registration is that a corner of the paper climbs one or both walls.
With higher speed finishing devices, this type of compiling does not keep up with the high production rate. An example of such a high speed finishing device is a newly introduced production finisher which operates at 157 ppm production rate. As the system speeds increase, a means to reduce finishing time without compromising stack registration is needed.
Accordingly, there is a need to provide a sheet registration and stacking system able to stack from one sheet up to a large number of sheets in sets with very close stack registration dimensions, both in the process direction and in the cross process direction.
There is a further need to provide a sheet registration and stacking system of the type described and that is able to stack and register sheets in the process direction and in the cross process direction simultaneously, so as to improve set registration and reduce the sheet compiling time, allowing sheets to be received at a faster rate without compromising in-set registration.
There is a yet further need to provide a sheet registration and stacking system of the type described and that is able to stack and register sheets rapidly, in the short time available between rapidly sequentially fed sheets, as in a high speed printer, so as not to slow down the sheet production rate of the printer.
There is a still further need to provide a sheet registration and stacking system of the type described and that is able to stack and register sheets with high reliability, absence of document edge damage or image smearing or operator danger. The system should accommodate a wide range of paper sheet sizes and weights and/or stiffness, and with an apparatus that is mechanically simple and robust, thereby minimizing cost and avoiding the problems associated with the prior art.